1. Field of the Invention
The present invention relates to an electronic odometer mounted on a vehicle for writing and reading data of integrated distance traveled using an EEPROM.
2. Description of the Prior Art
An electronic type odometer indicating an integrated distance traveled by a vehicle is arranged such that signals from a traveled distance sensor capable of generating the signal representative of number of revolutions of a wheel shaft or the like are counted and the integrated traveled distance obtained from the number of counts is indicated on a suitable digital indicator. Since the odometer uses the battery of the vehicle as its power source, it is necessary to supply it with electricity at all times so that the data of the integrated traveled distance may not be lost even when power supply to the counter portion is removed due to exchange of the battery or such trouble as line breakage, or the data may not be lost when the main power supply to the vehicle is turned off. To meet this necessity, it is practiced to write data of integrated traveled distance into a nonvolatile memory.
Under the present situations, however, there is a limit to the number of times of writing into a nonvolatile memory. To cope with this problem, there have been proposed various means such as that disclosed in the gazette published by the Patent Office of Japan for Laid-open Patent Publication No. 56-84512 wherein a counted from a preset counter is stored in a nonvolatile memory at the time when power is turned off. When the operation is resumed, the data from the memory is preset in the counter. Japanese Laid-open Patent Publication No. 56-154611 teaches counting by a C-MOS counter wherein a plus 1 is written into a nonvolatile memory by a carry generator each time a carry is generated while keeping the C-MOS counter supplied with power at all times. Japenese Laid-open Patent Publication No. 59-196414 teaches having the data of the integrated traveled distance sequentially written into a plurality of nonvolatile memories thereby updating the data every time a predetermined distance has been traveled and, when the operation is resumed, having all the data in the plurality of nonvolatile memories read out and the maximum value of them selected.
In writing integrated traveled distance data into a nonvolatile memory, the number of times of writing for updating data in a nonvolatile memory is reduced by the above described prior art, but there still remains a point somewhat problematic as to the realiability on the data obtained at the time of resuming operation. That is, supposing erroneous data are written in the nonvolatile memory, determination whether some data is erroneous or not will become more reliable if sets of data sufficiently large in number are written in the memory. In this sense, the number of sets of data written in the memory in the above described prior art is considered insufficient.
Hence, an invention was made as disclosed in the gazette published by the Patent Office of Japan for Laid-open Patent Publication No. 62-201312, in which, when employing a nonvolatile memory, the memory address was used as a portion of the integrated traveled distance data, thereby achieving reduction in number of times of writing data into the nonvolatile memory and enhancement of reliability on the data.